Don't allow JSON requests to trigger a new crawl or end a stuck one, since the requests may come too fast to handle.
Only allow triggering a new crawl if more than 'sp_timeout_crawl' seconds have passed since we canceled the previous one.
In the future, we might give each successfully initiated crawl a unique ID and then only allow sending a failure email once if it has failed. A very busy search engine is probably indistinguishable from a rapid-fire series of JSON requests.
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